The invention relates to a high pressure discharge lamp with first and second discharge devices connected electrically in series within an outer bulb, each discharge device including a discharge vessel enclosing a discharge space and an ionizable filling, first and second discharge electrode assemblies within the discharge space each including an electrode portion on which a discharge terminates during normal lamp operation and a current conductor portion extending to the exterior of the discharge vessel, and means for electrically connecting the first electrode assembly of each discharge device to a source of electric potential outside of the lamp envelope, and a starting aid facilitating ignition of the discharge devices.
Such a lamp is known from U.S. Pat. No. 4,751,432 (Van Delm). High pressure discharge lamps may have series connected discharge devices included within a single lamp envelope to decrease the overall size of the lamp or to achieve a blended light. Lumen output and consumed power of a high pressure discharge device are proportional to the physical separation between the discharge electrodes and consequently the overall length of the discharge device. Lamps rated at high power with a single arc tube therefore have a large overall length, which is generally undesirable from the optical as well as cost and handling standpoints. The overall length of the lamp can be significantly reduced, for example, by arranging two discharge devices within an outer envelope each operated at half the total desired power. Two discharge devices emitting different spectrums have also been employed to achieve an improved blended spectrum different from either device alone.
A high pressure arc discharge device is ignited by providing a starting pulse across the discharge electrodes with a prescribed voltage and bandwidth. This is typically accomplished with an external ignitor in a ballast contained in a lighting fixture. The ignition pulse(s) are applied through the lamp cap, usually in the form of a threaded base. Reliable ignition of such discharge lamps is frequently a problem, as the multiple discharge devices affect the starting characteristics of each other, generally requiring a starting pulse of much higher energy than that which reliably starts one discharge device of the same total wattage. However, safety constraints place an upper limit on the voltage of the ignition pulse applied through the lamp cap. Furthermore, commercial viability does not permit a lamp designer to market a lamp which requires its own special ballast and/or ignitor. Rather, HID lamps with multiple discharge devices rated at a certain total wattage are operated with existing ballasts designed to operate a lamp with a single discharge device of corresponding rated wattage.
The above-mentioned patent discloses a starting aid which sequentially starts the two discharge devices. The starting aid is a bi-metal switch which shorts one of the discharge devices to permit the ignition pulse to be applied initially across one device only. After the one device ignites and supports an arc discharge, the heat therefrom causes the bi-metal switch to open. This permits the ignition pulse to be applied across both the first and second discharge devices. Since the impedance across the already-burning discharge device is low, the second discharge device sees essentially the entire energy of the ignition pulse, providing reliable starting. One disadvantage of this construction is the long delay in igniting the second discharge device due to the time it takes for the first discharge device to heat the bimetal to its opening temperature, on the order of about 1-2 min. Additionally, bi-metal switches are cumbersome to install, usually requiring hand-mounting and/or adjustment